The weight of each case would be a factor to if this tip will work for you but maybe if you fold up a large lightweight duffle bag it will be lighter and give you a good deal of room to pack up extras after the cruise.
Yes, I do this. I just pack dirty laundry in the duffle leaving room in the more protective suitcase for purchases/more fragile items.
If I may, I'd like to suggest that a cruise trip may be a perfect time to try out Owner's Locker service. I've had one for a few years now, and I don't know what I did without it, especially when we would fly to Disney. Even though we are now within driving distance, I still would not (and did not) give up my locker - it's just too convenient! It's a perfect place to store "Disney-only" stuff (planners, ears, pins and lanyards, etc), as well as toiletries and even some convenient "extras" that we hesitate to pack each time, from ponchos, to lightsticks and for some, even libations and proper drinking glasses

. Storing those Disney-only items, shampoos, etc leaves more room (and weight) in your suitcases for those momentos we invaribly bring home with us.
DISboards has a special offer for the service, and the fee is satisfaction-guaranteed. If for any reason you want to cancel within the first year, they refund your money, no hassle (seriously, NO hassle - these guys have been GREAT to work with!!) "Try it, you'll like it!"
I know, I know - I sound like a commercial, but I don't work for the company and we are not related in any way, lol. (In interest of full disclosure, I can earn a gift card for referrals, but I did not provide my "real" info here, lest anyone think that is my motivation. If you do not list me as a referring source, I do not earn the card.) I just really, really love the service, and the price is well worth the convenience. And, you simply cannot beat the money-back offer. I don't think I have ever read a bad review on any of the Disney boards re: this service. I think everyone that has tried it (myself included) has first said "yeah, it's a nice idea, but do I really want to pay for it?", and everyone I know that DID try it has said something similar to "why did I wait?"